The present invention relates to an apparatus and a method for providing originals for polychromatic reproduction, particularly for preparing printing documents for polychromatic printing.
Printing with four colors which are printed on top of each other is widely used. In order to produce a spectrum of colors, suitable coverages of the chromatic primary colors cyan, magenta, and yellow and the achromatic primary color black are placed successively upon each image point of a substrate. Additional colors may be applied, such as gold, silver, lime-green, and others. Mixed colors can be produced by choosing appropriate amounts of coverage in two or more colorsxe2x80x94cyan and yellow, for example, combine to green.
Also used for polychromatic printing are screen printing and so-called flexographic printing which place several pure colors on a substrate successively.
In polychromatic printing the substrate to be printed on, preferably paper, passes several printing plates successively. The individual printing plates have to be aligned exactly with the moving substrate for objects or patterns to be placed in the desired position. In printing, as well as in similar processes for polychromatic reproduction, misregistrations may occur. Misregistrations in printing are attributed to inaccurate guidance of the substrate in the conveying apparatus as well as in perpendicular direction. It is known to compensate for such misregistrations by expanding the colors of individual color areas completely or partially into adjacent color areas. In this process color is placed on the substrate beyond the color areas originally provided. This process known as xe2x80x9ctrappingxe2x80x9d, is carried out according to predetermined criteria considering potentially adjoining colors.
In U.S. Pat. No. 5,481,379 a method for preparing polychromatic printing plates is described that is supposed to reduce such misregistration errors. In this system, first all pairs of picture elements are examined in which a color change occurs between adjoining color areas in at least one color separation. Subsequently, it is determined according to certain criteria whether a so-called frame should be made around a color area and which amount of coverage the frame should have in the different color separations. As examples, trapping rules are given, which determine whether for an existing color pair a trap is generated in the direction of one color. Furthermore combination rules are given, which determine the trap color of a frame.
In U.S. Pat. No. 5,542,052 a method is described, wherein first so-called trap areas are generated for individual color areas. Subsequently, parts of the area are removed from these proposed trap areas if they come into undesirably close proximity to other color areas or if they intersect existing trap areas.
It is an objective of the present invention to provide a device and a method to reduce registration errors in polychromatic reproduction, particularly in polychromatic printing.
This objective is attained by the features set forth in the appended claims.
The present invention is based upon the concept of analyzing color regions rather than boundaries as in the prior art. In a preferred embodiment of the method according to the invention, a distance mapping is carried out in one or a plurality of color regions within which traps are to be determined.